Passively mode-locked fiber lasers

ABSTRACT

A passive mode-locked linear-resonator fiber laser using polarization-maintaining fibers and a saturable absorber to produce ultra short pulses and a long-term reliable operation with reduced maintenance. Such a fiber laser can be configured to produce tunable pulse repetition rate and tunable laser wavelength.

This application claims the benefit of U.S. Provisional Application No.60/052,295 filed on Jul. 11, 1997, which is incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to fiber optical devices and lasers, andmore specifically, to mode-locked fiber lasers.

BACKGROUND OF THE INVENTION

Ultra short optical pulses can be used in a number of applicationsincluding optical information processing and data communication, opticalprobing with high temporal resolution, laser surgery, and materialprocessing. In particular, recent advances in optical data communicationwith data rates up to 2.5 Gbits/s or higher demand compact ultra fastlight sources with low maintenance, high reliability, and low cost.

Fiber lasers have been developed as a new generation of compact,inexpensive and robust light sources. In essence, a fiber laser is anoptically-pumped resonator with a doped-fiber as the gain medium. As thegain exceeds the total optical loss in the resonator, a laseroscillation can be generated. Many different dopants can be used toachieve laser oscillations at different wavelengths. Atomic transitionsin rare-earth ions can be used to produce lasers from visiblewavelengths to far infrared wavelengths (e.g., 0.45 μm˜3.5 μm). Er-dopedfiber lasers for producing optical pulses at 1.55 μm are particularlyuseful for optical fiber communication since the optical loss in thecommonly used silica fibers is minimum at about 1.55 μm.

Mode-locked fiber lasers can use various cavity is configurations suchas linear, ring, and figure-eight geometries. See, for example, U.S.Pat. Nos. 5,008,887 to Kafka et al., 5,513,194 to Tamura et al. Howeverconstructed, a mode-locked fiber laser is configured to have multiplelongitudinal modes that oscillate simultaneously. A mode-lockingmechanism is implemented in the resonator to synchronize the phases ofdifferent modes in such a way that the phase difference between any twoadjacent modes is a constant. These phase-locked modes constructivelyadd to one another to produce a short pulse.

Two common mode-locking schemes are active mode locking and passive modelocking. Active mode locking modulates either the amplitude or the phaseof the intracavity optical field at a frequency equal to one or amultiplicity of the mode spacing. Active mode locking can be implementedby using intracavity electrooptic and acoustooptic modulators.

Alternatively, passive mode locking uses at least one nonlinear opticalelement inside the resonator to produce an intensity-dependent responseto an optical pulse so that the pulse width of the optical pulse exitingthe nonlinear element is reduced. Compared to active mode locking,passive mode locking can be used to achieve shorter pulses and thereforecan be used advantageously to produce ultra short light sources.Commonly used passive mode locking techniques include saturableabsorbers, nonlinear fiber-loop mirrors (e.g., figure eight fiberlasers), and intensity-dependent nonlinear polarization rotation. See,Richardson et al., Electronic Letters, Vol. 1, pp. 542, 1991 and Tamuraet al., Electronic Letters, Vol. 28, 2226, 1992.

Mode-locked fiber lasers are much more compact and reliable thansolid-state mode-locked lasers such as color-center lasers andTi-Sapphire lasers. Compared to modelocked semiconductor lasers withtypical pulse widths of 10-20 ps and peak power of milliwatts,mode-locked fiber lasers can generate shorter pulses with higher outputpeak power.

SUMMARY OF THE INVENTION

The present disclosure includes a passive mode-locked fiber laser with asimple linear cavity and a saturable absorber to generate femtosecondpulses with a peak power up to and greater than tens of watts.

A mode-locked fiber laser according to one embodiment of the inventiongenerally includes an optical resonator defined by first and secondoptical reflective elements, a pump light source which provides a pumpbeam at a selected pump wavelength or within a specified pump spectralrange, a doped fiber gain medium disposed in the resonator responsive tothe pump beam to produce an optical gain at a laser wavelength within alaser gain spectral range, a pump optical coupler disposed to couple thepump beam into the doped fiber, and a saturable absorber disposedrelative to the second reflective element that produces anintensity-dependent absorption at the laser wavelength.

The doped fiber gain medium and other fiber links within the resonatormay be made of polarization-maintaining or polarizing fibers that arealigned with one another along a polarization axis. This keeps the laserpolarization parallel to a principle axis of the fibers without usingadditional polarization controlling devices. Thispolarization-maintaining configuration simplifies the construction ofthe resonator and allows for a reliable long-term laser operationwithout need for polarization maintenance.

The saturable absorber preferably exhibits a slow saturation process anda fast saturation process. The slow saturation process has a lowsaturation intensity and can be used to initiate mode locking when theintracavity intensity fluctuates at a low power level. As the pulseintensity builds up, the pulse width can be further reduced by the fastsaturation process. The slow saturation process allows use oflong-lasting low-power semiconductor light-emitting devices such as LEDsand laser diodes as the pump light source to achieve a reliableoperation up to the life time of these light sources. Many semiconductorcompounds have such slow and fast saturation processes that areoriginated from inter-band and intra-band transitions and can be used toimplement the invention.

The pump light source may include a light-emitting element such as a LEDand a laser diode to produce pump light at one or more pump wavelengthin resonance with at least one optical transition in the doped fibergain medium for producing photons at the laser wavelength. Thelight-emitting element can be electrically controlled to produce anadjustable output power.

The pump optical coupler may include a wavelength-division multiplexerthat couples the pump light into the doped fiber gain medium. The pumplight may be coupled into the resonator to propagate in a direction awayfrom the saturable absorber to reduce the amount of the pump light intothe saturable absorber, thus reducing any optical damage to the absorberby the pump light.

Wavelength-selective optical elements such as gratings and bandpassfilters can be incorporated into the resonator to effect a frequencytuning mechanism. This produces a tunable ultra short optical pulseswithin the gain spectral profile of the doped fiber medium.

A mechanism for tuning the pulse repetition rate can be further includedby changing the optical length of the resonator in a controllablemanner. A fiber stretcher or a positioner may be used for this purpose.In addition, a feedback loop may be implemented to lock the pulserepetition rate to an external clock. A portion of the output pulses isdetected by a photodetector. An error signal can be electronicallygenerated to indicate the relative delay of the pulse rate and theexternal clock rate. A control circuit sends a control signal to adjustthe length of the resonator by, for example, changing the position ofthe first reflective element, to reduce this error signal.

A fiber laser in accordance with the invention may be configured toproduce either transform-limited soliton pulses or non-soliton pulses.The soliton operation can be achieved by adjusting the cavity parametersso that the group-velocity dispersion and the nonlinear self-phasemodulation balance each other. Conversely, the laser may be adjusted toproduce non-soliton pulses as desired.

In addition, a fiber laser in according to the invention can beconfigured to significantly reduce noise and timing jitter in the outputpulses. Implementation of the polarization-maintaining configuration canreduce or minimize the noise and jitter caused by variations in thelight polarization. The reflection at the pump wavelength within theresonator can be reduced or minimized by using anti-reflection coatingat the pump wavelength on any optical surface, using angle-polishedfiber facets, or using bandpass filtering elements that block light atthe pump wavelength.

One advantage of the invention is the simplicity of the linear resonatorin a polarization maintaining configuration. Another advantage is thecapability of tuning the pulse wavelength. Yet another advantage is thecapability of tuning the pulse repetition rate.

These and other embodiments, aspects and advantages of the inventionwill become more apparent in light of the following detaileddescription, including the accompanying drawings and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing one embodiment of a polarization-maintaininglinear-cavity fiber laser.

FIGS. 2A, 2B, and 2C show different embodiments of a pump light source.

FIGS. 3A, 3B, 3C, 3D, 3E, and 3F show exemplary embodiments of the firstreflective element.

FIGS. 4A, 4B, 4C, and 4D show exemplary embodiments of the secondreflective element.

FIGS. 4E and 4F show two fiber lasers using all fiber components.

FIGS. 5A, 5B and 5C are charts showing measured pulse shape, spectralprofile, and pulse width v. wavelength of a fiber laser in accordancewith the invention.

FIG. 6 shows a tunable fiber laser with a feedback loop for locking thepulse repetition rate to an external clock.

FIG. 7 is a diagram showing an optical mount free of ball bearing.

FIGS. 8A, 8B, and 8C show one embodiment of a mount for holing a fibercollimator.

FIG. 9 shows a tunable fiber laser in a non-PM configuration.

FIG. 10 is a diagram showing an optical coupler using a beam splitter ina fiber-pigtailed configuration.

DETAILED DESCRIPTION OF THE INVENTION

Various embodiments of the invention are now described in detail withspecific reference to "fiber". Although the term "fiber" is in generalunderstood as an optical fiber comprising a center core and an outerportion that contains an optical beam, "fiber" is used throughout thisdisclosure to include any optical waveguiding conduit such as opticalfibers and waveguides.

FIG. 1 is a diagram showing one embodiment 100 of the mode-locked fiberlaser in a linear cavity configuration. The fiber laser 100 has anoptical resonator formed of a first reflective element 110, a fiber gainmedium 130, a fiber optical coupler 140 for coupling a pump beam to thefiber gain medium 130, and an optical module 120 having a saturableabsorber 125 for mode locking and a second reflective element 126 toprovide optical feedback for laser oscillation.

A means for coupling the laser pulses out of the resonator as the outputof the fiber laser 100 can be implemented in a number of ways. FIG. 1shows a fiber output coupler 160 for producing the laser output in whichan optical isolator 170 may be used to reduce any optical feedback tothe laser resonator. Alternatively, the laser output may be implementedby making a reflector at one end of the resonator, e.g., the reflector126, partially transmissive at the laser wavelength (e.g., a fewpercent) or by using a beam splitter to reflect a small fraction of theintracavity beam out of the resonator.

The fiber gain medium 130 is a doped fiber segment with desired opticaltransitions for laser oscillation. One class of commonly used dopants israre-earth ions such as erbium, holmium, neodymium, samarium, thuliumand ytterbium. A suitable pump wavelength is usually at a wavelengthshorter than the laser wavelength due to the atomic transitions in mostdopants. For example, Er⁺³ ions can be doped in silica/fluoride fibersto produce laser oscillations at 1.55 μm by using the laser transition ⁴I_(11/2) →⁴ I_(15/2) and a pump beam at 980 nm or 1480 nm. An Er-dopedfiber is an optical gain medium known for the high efficiency. A typicalEr-doped fiber can produce a gain of up to or greater than 20 dB withseveral milliwatts of pump power at 980 nm or 1480 nm.

In addition, two or more different rare-earth ions can be mixed togetherto achieve certain pump and laser wavelengths that may not be readilyavailable from a single doping element. For example, erbium ions andytterbium ions can be doped in silica fibers at some relative proportionto produce laser oscillations near or at 1.5 μm for a pump wavelength atabout 1.05 μm. Such doped fibers can be advantageously used as gainmedia to use pumping light sources around 1.05 μm (e.g., diode-pumpedYAG lasers) since these light sources are well developed andcommercially available at a relatively low price.

Polarization-maintaining single-mode ("PM") fibers or polarizingsingle-mode fibers are preferable for any fibers in the optical path ofthe laser pulses, i.e., the doped fiber 130 and other undoped fibersegments linking various optical elements in the resonator. Fibersegments 140a, 140b, 160a, 160b, and 121a for linking optical couplers140, 160 and optical collimator 121 are such examples. Preferably, fibersegments 144 and 164 in the couplers 140 and 160 are also PM orpolarizing fibers although regular fibers may be used. Apolarization-maintaining fiber is configured to have well-definedprinciple axes for two mutually orthogonal polarizations. A polarizingfiber has a single principle polarization axis. These two types offibers can be configured so that a principle axis is essentially notinfluenced by environmental conditions, such as fiber position,temperature, and stress. Therefore, the polarization of a beampropagating in such a fiber can be maintained. In the followingdescription, "polarization-maintaining fiber" fiber will be used toinclude any fiber or optical waveguide that can preserve an opticalpolarization of a beam in a resonator.

All polarization-maintaining fiber segments in the fiber laser 100should be aligned so that the respective principal axes of differentfiber segments are parallel with respect to one another. This forms apolarization-maintaining construction that maintains the polarization ofoptical pulses along a principle axis of the polarization-maintainingfibers. As the laser oscillation is building up from the spontaneousemission in the resonator, a principle axis with the minimum lossdefines the polarization of the laser output. This polarizationmaintaining property is only needed to maintain the polarization oflight at or near the laser wavelength and may not be required at thepump wavelength.

The fiber laser 100 of FIG. 1 provides an optical resonator to build anelectromagnetic field due to the presence of optical gain medium andoptical feedback. The electrical field distribution inside the resonatormust satisfy a resonance condition to generate a desired laseroscillation. One important aspect of this resonance condition is thestate of the polarization of the electrical field inside the resonator,which can affect the feedback mechanism of the laser resonator. At leasttwo polarization states are generally possible in a laser resonator.Only one polarization state can be selected and maintained for lasing toachieve a stable laser oscillation since two or more polarization statescompete with one another for lasing and may cause adverse chaotic lasingoperations. The fiber gain medium 130 and other fiber segments in thelaser cavity provide an optical conduit in which the electrical fieldsatisfying the resonance condition is distributed. Therefore, thesefibers should maintain the selected polarization state.

Conventional single-mode fibers are not required to be polarizationmaintaining because many optical communication fiber devices and systemsand other applications use optical fibers to simply transmit opticalenergy rather than an optical field. Such a conventional single-modefiber does not have well-defined polarization axes for the twopolarization modes supported by the fiber. The axes of the polarizationcan change with environmental conditions such as fiber position,temperature, and stress. As a result, the polarization of light in aconventional fiber varies upon the environmental change and the passageof time, often in a unpredictable manner. Therefore, one or morepolarization control elements are usually implemented in fiber lasersthat use non-PM fibers. However, polarization control elements oftenrequire optical alignment with high precision and complex diagnosisequipment for determining the polarization state. The need for periodicmaintenance of the alignment of these polarization control elements canseverely limit the utility of fiber lasers in industrial and commercialapplications such as massive communication systems and manynon-optical-field applications.

The polarization-maintaining laser configuration using PM fiberseliminates the need for maintenance of the polarization state. The fiberlaser 100 can be designed and constructed using PM fibers to achieve anessentially maintenance-free laser operation. A polarization-selectingcomponent such as a polarizer, or a grating can be easily placed in thelaser resonator to eliminate the unwanted polarization state(s).

Referring to FIG. 1, a pump light source 150 produces a pump beam at oneor more desired pump wavelengths within a suitable pump wavelength rangebased on the dopant used in the fiber gain medium 130. For Er-dopedsilica fibers to effect laser oscillations at or near 1.55 μm, one ormore allowed wavelengths for the pump light source 150 are approximatelywithin a range from 965 nm to 995 nm. Any light-emitting de vice may beused in the pump light source 150, including a semiconductor diodelaser, a light-emitting diode, or a wide spectral light source with anarrow bandpass filter. A fiber segment 142 is used to guide the pumpbeam from the pump light source 150 into the resonator.

FIGS. 2A, 2B, and 2C show three exemplary configuration of the pumplight source 150. An electrically driven light-emitting element 152(e.g., a LED or laser diode) is coupled to the fiber 142. An electronicdriver 154 connected to the light-emitting element 152 provides adriving current and a pump power control. An optical control element,such as a wavelength-selective attenuator 156 formed of a fiber loopwhich transmits light at the pump wavelength and attenuates light at thelaser wavelength as in FIG. 2A, an optical isolator 157 as in FIG. 2B,or a variable attenuator 158 as in FIG. 2C, may be disposed in theundoped fiber segment 142 to reduce any optical feedback to the opticalresonator. These elements reduce laser noise and improves the laserstability.

The pump optical coupler 140 is disposed relative to the doped fibersegment 130 to couple the pump beam in the fiber 142 into the dopedfiber segment 130. Two undoped fiber segments 140a and 140b may be usedto optically coupled the coupler 140 into the resonator. The fiber 140ais connected to one end of the doped fiber 130 by, for example, fusionsplicing. The pump beam is directed into the doped fiber 130 to producepopulation inversion at a desired optical transition. This initiatesspontaneous emission at the desired laser wavelength defined by theoptical transition and build up the laser oscillation due to the opticalfeedback as the output power of the pump light source 150 exceeds athreshold level. Since the laser wavelength is different from the pumpwavelength, the pump optical coupler 140 should be a wavelength-divisionmultiplexer ("WDM"). For Er-doped fiber laser, the WDM 140 is configuredfor coupling energy between two different channels near a 1.55-μm lasingregion (e.g., from 1520 nm to 1580 nm) and near a 0.98-μm pump region(e.g., from 965 nm to 995 nm).

The optical pumping geometry shown in FIG. 1 is designed to increase thepumping efficiency and to reduce the amount of the pump power at thepump wavelength that reaches the saturable absorber 125. This extendsthe life of the saturable absorber 125 and improves the laser operation.More specifically, the WDM 140 is placed in the optical path between thedoped fiber medium 130 and the saturable absorber 125 to couple the pumpbeam from the pump light source 150 into the doped fiber medium 130.Thus, a majority of the pump power is absorbed by the doped fiber medium130. A fraction of the pump beam transmitted through the doped fibermedium 130 is reflected towards the saturable absorber 125 by the firstreflective element 110. The first reflective element 110 can be designedto further reduce the reflected pump power towards the saturableabsorber 125. This may be achieved by, for example, using anangle-polished fiber end facet, an anti-reflection coating for the pumpwavelength, or implementing a bandpass filter that absorbs light at thepump wavelength and transmits the light at the laser wavelength.

The first reflective element 110 as shown in FIG. 1 includes areflective grating 112 and a fiber collimator 114 that is coupled to oneend of the doped fiber segment 130 through an undoped fiber segment 111.Alternatively, the collimator 114 may be directed coupled to the dopedfiber 130. The grating 112 is rotatably mounted at a fixed positionrelative to the collimator 114 so that the grating 112 can be rotated tochange the grating Bragg condition to select a desired laser wavelengthfor optical feedback. Optical signals at wavelengths that do not satisfythe Bragg condition are attenuated by the grating 112 and thus cannotbuild up to form laser oscillations. Therefore, the grating 112 providesa frequency tuning mechanism for tuning laser frequency within the gainspectral range of the doped fiber 130. The grating angle adjustment maybe implemented by using an optical mount, rotating positioner or thelike.

In addition, the grating 112 may also be used to maintain thepolarization of the optical pulses by aligning the grating grovesparallel or perpendicular to the preferred principle axis of the PMfibers.

The saturable absorber 125 provides a mode-locking mechanism in thelaser 100. The absorption of the saturable absorber 125 is dependent onthe optical intensity in such a way that the absorption coefficient isless at high intensities than that at low intensities. Hence, theintensity fluctuation in the laser resonator can be sharpened by thesaturable absorber 125 to generate short optical pulses. As the laseroscillation reaches a steady state, stable optical pulses with aconstant pulse repetition rate are formed. This use of a saturableabsorber for mode locking is well known. The saturable absorber 125 maybe formed by a number of materials such as chemical dyes, polymers orsemiconductor materials (e.g., quantum-well structures). In general, thesaturable absorber 125 can be placed anywhere within the laser resonatorbut the position as shown in FIG. 1 is preferable.

Semiconductor saturable absorbers with bandgaps not exceeding the photonenergy corresponding to the laser wavelength are preferred inimplementing the invention. For an Er-doped fiber as the fiber medium130 to produce laser pulses at 1.55 μm, a suitable semiconductorsaturable absorber should have a bandgap equal to or less than thephoton energy at 1.55 μm. Hence, InGaAs or InGaAsP may be used as thesaturable absorber 125. The semiconductor saturable absorber 125 may bein contact with the reflecting surface of the second reflective element126 or may be integrated to the second reflective element 126 as in amultiple quantum well structure that has a quantum-well Bragg reflector.

A semiconductor saturable absorber may need to undergo certain materialprocessing steps to exhibit desired saturable absorptioncharacteristics. It has been found that optical damage, ionimplantation, or low-temperature growth can be used for this purpose.Lin et al. disclose the ion implantation method in U.S. Pat. No.5,436,925, which is incorporated herein by reference. A desired range ofthe linear absorption (e.g., the small signal absorption for opticalintensity much smaller than the saturation intensity) of the saturableabsorber 125 is from approximately 2% to approximately 90%, and morepreferably from approximately 25% to approximately 75%.

A semiconductor saturable absorber is preferred in practicing theinvention partially because a special characteristics can be used torealize self-initiated mode locking. A semiconductor saturable absorbercan have both intra-band and inter-band transitions. The inter-bandprocess has a slower response and a lower saturation energy than theintra-band process. Thus, the slow saturation process initiates the modelocking even when the intracavity intensity fluctuates at a low powerlevel. As the pulse intensity increases and the pulse width reduces, thepeak pulse intensity saturates the semiconductor absorber to is effectthe fast saturation process which further sharpens the pulses. Thisunique feature can be used for a "turn-key" operation of the fiber laser100 in the sense that no additional operation other than turning on thepump light source 150 is necessary to achieve mode-locked pulsed laseroscillations.

Using a semiconductor absorber also allows for the use of low-powerdiode lasers of several milliwatts to tens of milliwatts as the pumplight source 150 since the slow saturation inter-band process caninitiate mode locking at low intracavity intensities. This isparticularly important in practical devices and systems since low-powerdiode lasers used for telecommunication (e.g., near 980 nm or 1480 nm)can have a life time up to or longer than 200,000 hours. The fiber laser100 with such a low-power diode pump source can be operated for the lifetime of the pump diode laser. For optical data communicationapplications, this significantly reduces the down time and maintenance.As a comparison, a fiber laser based on a high-power diode pump laser ofseveral hundred milliwatts may need to replace the diode pump laserevery several hundred hours. In addition, low-power diode lasers areless expensive than high-power diodes, reducing the manufacturing cost.

Optical nonlinear effects in fibers may be also used to induce theeffect of the saturable absorption of a saturable absorber for modelocking. However, such nonlinear effects demand high opticalintensities. The preferred operation of the fiber laser 100 uses pumppower much lower than the threshold level of the fiber nonlinearity(about 10² Watts) to avoid such effect but sufficient to generate lightat the laser wavelength from the fiber gain medium to saturate thesemiconductor saturable absorber. Use of the semiconductor salurableabsorber also avoids the adverse residue CW lasing in mode locking usingonly fiber nonlinear effects.

A collimator 121 and a lens 123 are disposed in the module 120 to couplethe optical energy between the fiber segment 121a and the saturableabsorber 125. The collimator 121 shapes the optical beam from the fiberinto a substantially parallel beam. The lens 123 is positioned withrespect to the saturable absorber 125 so that the spacing therebetweenis about the focal length of the lens 123 with an adjustable rangewithin the Rayleigh length of the intracavity optical pulses.Preferably, the lens 123 focuses the beam onto the saturable absorber125 with a spot size down to and/or smaller than about 5 μm in diameter.

Another aspect of the fiber laser 100 in FIG. 1 is a tuning mechanismfor adjusting the pulse repetition rate. This tuning mechanism can beachieved by changing the cavity length of the resonator, i.e., theoptical length between the grating 112 and the second reflective element126. For example, a fiber segment may be stretched or the distance of acollimated beam may be changed such as the air-gap distance between thelens 123 and the collimator 121. When the fiber laser 100 is configuredto have one laser pulse per a round trip time, the round trip timewithin the resonator is the temporal separation between two adjacentpulses. Under this condition, changing the cavity length effects achange in the pulse repetition rate. A number of techniques may be usedto change the cavity length. For example, a positioner, such as amechanical translation stage or a piezo-driven positioner, may beengaged to the first or the second reflective element to change thecavity length. Specifically, such a positioner can be used to change therelative position of the grating 112 with respect to the collimator 114.A fiber stretcher such as a piezo fiber stretcher can be disposed in anyfiber segment to change the cavity length by applying an electricalcontrol signal to the piezo.

FIGS. 3A-3F show alternative embodiments of the first reflective element110. FIG. 3A uses a lens 114a in place of the collimator 114 in FIG. 1.The lens 114a is placed away from the grating 112 with a spacing largerthan the lens focal length f to image the free end facet of the fiber111 to the grating 112. Since the beam is a Gaussian beam, the beam onthe grating surface is essentially collimated within the Rayleigh lengthrange. The output facet of the fiber 111 as shown is perpendicular tothe fiber core. This facet is preferably coated with an anti-reflectioncoating at the pump wavelength to reduce the reflection of the pump beamcoupled into the resonator from the coupler 140. This coating may alsobe anti-reflective at the laser wavelength to reduce a sub-cavity effectdue to intra-cavity reflection at the laser wavelength. FIG. 3B issimilar to FIG. 3A except that the output facet 111a of the fiber 111 ispolished at an angle of several degrees (typically, about 6°˜8°) toreduce optical reflection at both laser wavelength and pump wavelength.The angle-polished fiber 111 is tilted at an angle with respect to theoptic axis of the lens 114a in order to achieve proper optical coupling.FIG. 3C shows an embodiment that uses a reflector 113 with a highreflectivity at the laser wavelength and a low reflectivity at the pumpwavelength. FIG. 3D places a lens 114a away from the output facet of thefiber 111 by a focal length to collimate the beam incident to the highreflector 113. This optical configuration can also replace the focusingconfiguration shown in FIGS. 3A-3C.

FIG. 3E shows an embodiment of the first reflective element 110 thatuses a bandpass filter 116 to tune the laser wavelength within the gainspectral profile of the fiber gain medium 130. The filter 116 may be aninterference filter with a transmission wavelength as a function of theangle. Thus, the laser wavelength within the gain spectral range of thelaser doped fiber 130 can be selected by adjusting the filter angle. Thereflector 113 may be displaced with respect to the collimator 114 by apositioner to change the cavity length, thereby the pulse repetitionrate. Other bandpass filters such as birefringence filter may also beused. Furthermore, a reflective fiber grating may be used as the firstreflective element 110. The fiber grating may be configured to have atuning mechanism to change the resonant Bragg condition such as thegrating spacing (e.g., fiber stretching by a piezo element) or theeffective index of refraction. This tunes the resonant wavelength of thefiber grating, thus changing the laser wavelength.

The above optical bandpass filtering to select the laser wavelength maybe implemented by using a fiber pigtailed bandpass filter which may usesan interference filter, a birefringence filter, or a grating to tune thelaser wavelength. Such a fiber pigtailed filter may be convenientlydisposed anywhere in a fiber laser and can be tuned manually orelectronically. The connecting fiber segments on such a filter should bethe PM type when a polarization-maintaining configuration is desired.

FIG. 3F further shows another embodiment of the first reflective element110 in which the high reflector 113 is directly formed over a cleavedfacet of the fiber 111. A layer of adhesive 113a may be used to form thedirect contact between the reflector 113 and the fiber facet. The facetis perpendicular to the fiber core so that the reflecting surface of thereflector 113 is also perpendicular to the fiber core. A supportingstructure 119 has a through hole that is sized to tightly hold the fiber111 at a desired position. This design can be easily manufactured at lowcost. Alternatively, a reflecting coating may be directly applied to thefacet of the fiber 111 to function as the first reflective element.

The fiber laser 100 may include an optional variable attenuator 122 asshown in FIG. 4A in the resonator to control the intracavity opticalintensity to improve the mode locking by the saturable absorber 125. Apolarizer 124 aligned with a specified polarization axis of the PMfibers may also be placed in the resonator to further ensure the properpolarization of the laser pulses. When the fibers have two orthogonalprinciple axes, the polarizer 124 eliminates the possibility of laseroscillations in two polarization states. FIG. 4A shows one possibleconfiguration for the second reflective element 120 that includes theattenuator 122 and the polarizer 124.

FIGS. 4B and 4C show additional embodiments of the second reflectiveelement 120. A lens 121b is placed between the fiber 121a and thesaturable absorber 125 to couple the optical energy to and from thesaturable absorber 125 by imaging. FIG. 4B shows the end facet of thefiber 121a is angle-polished to reduce the effect of the reflection.

The construction of the unit 120 as shown in FIGS. 1 and 4A-4C may besimplified by directly coupling the free end of the fiber 121 to thesaturable absorber 125. FIG. 4D illustrates such a construction. Thisconstruction eliminates optical elements such as collimator 121 and lens123 and respective optical alignment. For a single-mode fiber for 1.55μm, the beam spot size on the saturable absorber 125 may beapproximately from 3 μm to 10 μm in diameter.

A tunable "all-fiber" fiber laser can be constructed by combining afiber grating or a fiber pigtailed bandpass filter with the secondreflective element in FIG. 4B. FIGS. 4E and 4F show two exemplaryall-fiber laser configurations 401 and 402.

The fiber laser 401 in FIG. 4E uses a fiber grating 410 as the firstreflective element to provide optical feedback and frequency tunability.A fiber grating controller 412 controls the Bragg condition of the fibergrating 410 to tune the reflected wavelength. One example of thecontroller 412 is a piezo fiber stretcher which changes the gratingperiod: the laser wavelength increases with the grating period. Thesecond reflective element 120 is the same as the embodiment in FIG. 4D.

The fiber laser 402 in FIG. 4F uses the same construction for the secondreflective element 120 as in the fiber laser 401 FIG. 4E but a differentconfiguration for the first reflective element 110. Specifically, atunable pigtailed bandpass filter 420 is connected to the fiber gainmedium 130 by, for example, fusion splicing. The other fiber 420b of thefilter 420 is directly coupled to a high reflector 113 to form aconstruction similar to FIG. 3F. Alternatively, the high reflector 113may be a reflective coating formed on the facet of the fiber 420a. Inaddition, the pigtailed bandpass filter 420 may be placed at any otherfiber section within the resonator.

A fiber stretcher can be disposed in any fiber segment in the lasers 401and 402 to tune the pulse repetition rate by changing the total opticallength of the laser resonator. One location for the fiber stretcher canbe the fiber gain medium 130.

A jittering element may be further included in a fiber laser tofacilitate the mode locking. For example, the aforementionedpiezo-driven fiber may also be used to induce a disturbance in theresonator. As a control voltage is applied to the piezo, the action ofstretching in the portion of the fiber on the piezo induces adisturbance to initiate the mode locking.

A further aspect of the invention is reduction of noise and timingjitter in the output pulses. The polarization-maintaining configurationby using PM fibers essentially eliminate the noise and jitter caused byvariations in the light polarization. But unwanted optical feedbacksignals can also cause the laser wavelength and intensity to fluctuate,or even cause a fiber laser to lase chaotically. In addition, unwantedfeedback can lead to a breakdown of an established mode-lockingoperation. The unwanted optical feedback signals can be reduced in anumber of ways.

For example, the first reflective element 110 can be designed to reflectonly the light at the laser wavelength and to significantly reduce oreliminate any light reflection at the pump wavelength. This reduces theadverse feedback to the pump light source 150. One implementation usesan angle cleaved or angle polished facet (e.g., larger than 6°) at thefree end of the PM fiber 140a of the WDM coupler 140. Anotherimplementation applies an anti-reflection coating designed for the pumpwavelength on the facet of the facet at the free end of the PM fiber140a of the WDM coupler 140.

For another example, unwanted feedback to the mode-locked fiber lasercan be reduced by using wavelength selecting attenuating element asshown in FIGS. 2A-2C or the isolator 170 in the output fiber 162. Thecollimators 121, 114 and any lens (e.g., lens 123), and any lighttransmitting surface of the fiber laser 100 in FIG. 1 may be designed tohave a significantly reduced reflectivity, e.g., less than 10⁻⁵. Thedesigns shown in FIGS. 3A-F can also be used to reduce adversereflections in the laser resonator. The free end of the output fiber 162can also be cleaved at an angle to reduce reflection. A reflectingsurface (e.g., a reflective grating or a mirror) in the laser resonatorshould have a high reflectivity at the laser wavelength and a lowreflectivity at the pump wavelength.

A fiber laser in accordance with the invention may be operated either ina soliton mode or in a non-soliton mode. Soliton is a special nonlinearphenomenon in which an optical pulse maintains its shape and spectralprofile essentially unchanged during propagation in fibers. An opticalpulse traveling in a fiber is subject to the fiber dispersion so thatdifferent frequency components in the pulse travel at different groupvelocities. This dispersion causes pulse broadening in the time domain.In addition, a pulse also experiences a nonlinear effect "self phasemodulation" ("SPM") caused by the intensity dependence of the refractiveindex of the fiber. SPM can lead to new frequency components in highintensity pulses, thus effectively broadening the pulse in the frequencydomain. Soliton pulses are generated from a fiber laser when the fiberdispersion is negative, i.e., the group velocity of a high frequencycomponent is higher than that of a low frequency component, and thegroup velocity dispersion compensates for SPM.

Soliton pulses are desirable in certain applications such as opticalcommunication systems since a soliton pulse is transform-limited (i.e.,the cleanest frequency components for a given pulse width) and has lowpedestals and noise. In addition, soliton effects can be used for pulsecompression so that a mode-locked soliton fiber laser can include apulse compression module to produce soliton pulses with a furtherreduced pulse width. It has been found that several parameters in theabove PM fiber lasers can be adjusted to achieve a soliton operation.For example, the power level and the total group velocity dispersioninside the resonator can be adjusted to substantially cancel the effectsof SPM. This configuration results in soliton pulses.

The soliton configuration can produce transformed limited pulses ofabout 0.5 ps with a time-bandwidth produce of about 0.32 by using a1550-nm Er-doped PM fiber laser. FIG. 5A is the auto correlation traceof such an Er-doped PM fiber laser similar to the embodiment 100 in FIG.1 except that the configuration shown in FIG. 3A is used as the firstreflective element 110. The auto correlation is measured by mixing twobeams that undergo different time delays and are split from the laseroutput in a nonlinear crystal to produce a second harmonic generationsignal. The vertical axis represents the intensity of the autocorrelation trace. The typical relative intensity noise of this laser isabout -120 dBc/Hz and a timing jitter integrated to 100 Hz is about 0.2ps. FIG. 5B is a measured output spectrum of a laser pulse. The verticalaxis is in a log scale with 5 dB per division. Each division in thehorizontal axis represents 5 nm. The center wavelength of the pulse isabout 1545 nm and the full width at the half maximum is about 5.31 nm.The side spectral peaks indicate the pulse is a soliton pulse. FIG. 5Cfurther shows measured pulse width as a function of the wavelength. Thepulse width remains below 900 fs as the laser is tuned from about 1525nm to about 1568 nm by using the grating.

Alternatively, a fiber laser can also operate in a non-soliton mode asdesired by setting the parameters in such a way that the group velocitydispersion and SPM are not balanced.

A PM fiber laser according to the invention can be configured to havetunable pulse rate by changing the optical length of the resonator. Thisfeature can be used to lock the pulse rate to an external clock. FIG. 6shows such a system 600. A photo detector 610 receives at least aportion of the output from a fiber laser 601. A pulse-locking circuit620 measures the pulse rate based on an electrical signal from thedetector 610. The circuit 620 further receives a reference clock signalfrom an external clock generator to which the pulse rate is to belocked. The pulse rate and the reference clock are compared by thecircuit 620 to produce a clock error signal. The magnitude of this clockerror signal indicates the amount of relative delay between the pulserate of the laser 601 and the reference clock. The sign of the errorsignal indicates the direction of the relative delay. A cavity lengthcontroller 640 adjusts and maintains the cavity length according to theerror signal in such a way that the error signal is reduced to a smallvalue within a specified tolerable range.

The cavity length controller 640 may be implemented in a number ofconfigurations. For example, one implementation uses a piezo fiberstretcher with a voltage power supply. Another implementation uses anelectrically controllable positioner (e.g., driven by a step motor orpiezo transducer) to change the relative position between the reflector113 and the collimator 114.

The combination of the polarization maintaining fiber configuration, thesemiconductor saturable absorber, and the simple linear cavity resultsin a fiber laser in accordance with the invention capable of producingultra short lasers pulses with a turnkey operation, long termreliability, significantly reduced maintenance (even possibly nomaintenance). Several adjustable optical mounts have been contemplatedand used to further improve the reliability of such fiber lasers.

FIG. 7 shows an mount 700 suitable for holding the assembly of thesaturable absorber 125 and the second reflective element 126. Two rigidplates 710 and 720 made of a rigid material (e.g., aluminum or steel)are held together by three adjustable screws 730a, 730b, and 740. Screws730a and 730b are engaged to the plate 710 by threaded holes and to theplate 720 by through holes. Screw 740 is engaged to the plate 720 by athreaded hole and to the plate 710 by pressing against one end tocentral spot. The plate 720 is fixed relative to the resonator so thatscrews 730a and 730b controls the orientation of the plate 710 and thescrew 740 controls the relative spacing from between the plates 710 and720. This design eliminates the conventional spring-loaded ball bearingconfiguration for many mirror mounts and can be used to achievelong-term stability to ensure proper alignment.

FIG. 8A shows a side view of an adjustable mount 800 for holding theoptical collimator 114 and 121 in the fiber laser 100 in FIG. 1. Themount 800 has a base 810 with a first base part 810a and a second basepart 810b in a shape of the letter "L". The base 810 is made of a rigidmaterial such as aluminum or steel. A slot 812 is formed in the firstbase part 810 to change the position of the second base part 810b uponbeing compressed by a set of screws 814a and 814b. Screws 814a adjustthe position of the second base part 810b in a direction opposite tothat controlled by screws 814b. A slot 812b is formed in the second basepart 810b to split a portion thereof to form an adjustable part 810c.The second base part 810b has a through hole 820 to hold a collimator802. Screws 816 on the adjustable part 810c are used to adjust therelative tilt of the part 810c in order to control the orientation ofthe collimator 802. FIGS. 8B and 8C show different side views of themount 800 along the lines 8B--8B and 8C--8C, respectively. This designcan also be used to eliminate the conventional ball bearing.

Although the present invention has been described in detail withreference to a few embodiments, various modifications and enhancementsmay be possible. For example, a tunable linear-cavity fiber laser may beconstructed by using non-PM fibers according to the invention. Anexample 900 is shown in FIG. 9 in which an adjustable positioner 920 isengaged to the grating 112 for tuning the pulse rate. A fiberpolarization controller 910 is added in the resonator to maintain thelight polarization since the fibers are not the PM type. An optionallinear polarizer 124 may also be used.

The WDM 140 and output coupler 160 may also be formed by using bulkoptical elements such as a beam splitter. FIG. 10 is one embodiment 1000in a fiber pigtailed configuration. A beam splitter 1010 is opticallycoupled to three fiber collimators 1020a, 1020b, and 1020c. Fibersegments 1040a and 1040b are connected in the resonator and fibersegment 1030 is used for input or output. To use the device 1000 as anoutput coupler, the beam splitter 1010 is coated with a low reflectivity(e.g., about several percentage) at the laser wavelength and the fiber1030 produces the output. To use the device 1000 as a pump coupler, thebeam splitter 1010 is dichroic, i.e., coated to have a high reflectivityat the pump wavelength and to be anti-reflective at the laserwavelength.

Gratings of the reflective type have been described in variousembodiments to tune the laser wavelength. However, a transmissivegrating may also be used to select a desired laser wavelength tooscillate in the resonator.

These and other variations are intended to be encompassed by thefollowing claims.

What is claimed is:
 1. A fiber laser having a linear optical resonator, comprising:a fiber gain medium having optical transitions operable to absorb optical carriers at a pump wavelength of a specified wavelength range and to emit optical carriers at a laser wavelength that is different from said pump wavelength; first and second reflective elements disposed relative to said fiber gain medium to form a linear optical resonator that encloses said fiber gain medium and supports a plurality of longitudinal modes; a saturable absorber disposed in said optical resonator and configured to exhibit an intensity-dependent absorption, said saturable absorber operable to effect a mode-locking mechanism that locks said longitudinal modes in phase to produce optical pulses at said laser wavelength; a pump light source, operable to produce a pump beam at said pump wavelength which optically excites said fiber gain medium; an optical coupler, disposed relative to said fiber gain medium to couple said pump beam into said fiber gain medium; and a tuning element located in said optical resonator and configured to change either the optical length of said optical resonator or said laser wavelength.
 2. A fiber laser as in claim 1, wherein said fiber gain medium is formed of a polarization-maintaining fiber having one polarization axis that defines the polarization of said optical pulses.
 3. A fiber laser as in claim 1, wherein said tuning element includes a grating that is operable to tune said laser wavelength within a gain spectral range of said fiber gain medium.
 4. A fiber laser as in claim 1, wherein said tuning element includes a bandpass filter that transmits photons at said laser wavelength and absorbs photons at said pump wavelength, said bandpass filter operable to tune said laser wavelength.
 5. A fiber laser as in claim 1, wherein said fiber gain medium is doped with rare-earth ions.
 6. A fiber laser as in claim 5, wherein said rare-earth ions include Er ions, said laser wavelength is about 1.55 μm.
 7. A fiber laser as in claim 1, wherein said saturable absorber includes one or more semiconductor compounds each having a bandgap equal to or less than a photon energy corresponding to said laser wavelength.
 8. A fiber laser as in claim 7, wherein said one or more semiconductor compounds are configured to have a linear absorption at said laser wavelength from about 25% to about 75%.
 9. A fiber laser as in claim 1, wherein said optical coupler is positioned in an optical path between said fiber gain medium and said saturable absorber to couple said pump beam into said fiber gain medium by propagating from said optical coupler to said fiber gain medium.
 10. A fiber laser as in claim 1, wherein said saturable absorber is configured to have a slow saturation process with a low saturation intensity and a fast saturation process with a high saturation intensity so that said optical pulses are initiated by said slow saturation process and further modified by said fast saturation process.
 11. A fiber laser as in claim 1, further comprising means for reducing light at said pump wavelength that reaches to said saturable absorber.
 12. A fiber laser as in claim 1, wherein said tuning element includes a fiber stretcher engaged to said fiber gain medium, said fiber stretcher operable to change a repetition rate of said optical pulses.
 13. A fiber laser as in claim 1, wherein said tuning element includes a positioner engaged to one of said first and second reflective elements to change said optical length between said first and second reflective elements, thus changing a repetition rate of said optical pulses.
 14. A fiber laser as in claim 1, further comprising an output optical coupler disposed in said resonator to produce an output.
 15. A fiber laser as in claim 14, wherein said output optical coupler includes a fiber segment to carry said output.
 16. A fiber laser as in claim 15, wherein said output optical coupler includes an optical isolator to reduce a feedback to said optical resonator.
 17. A fiber laser with a linear optical resonator, comprising:first and second reflective elements disposed relative to each other to form a linear optical resonator having a plurality of longitudinal modes; a fiber gain medium having optical transitions operable to absorb photons at a pump wavelength in a specified wavelength range and to emit photons at a laser wavelength that is different from said pump wavelength, said fiber gain medium configured to maintain light polarization in a specified direction perpendicular to an optic axis of said fiber gain medium; a semiconductor absorber disposed in said optical resonator and configured to exhibit a saturable intensity-dependent absorption and to have a bandgap equal to or less than a photon energy corresponding to said laser wavelength, said semiconductor absorber operable to lock a plurality of oscillating longitudinal modes in phase to produce optical pulses of said laser wavelength at a pulse repetition rate; an optical coupler, disposed relative to said fiber gain medium and configured to couple a pump beam at said pump wavelength to said fiber gain medium; and a plurality of polarization-maintaining fiber segments having a principle axis aligned with said specified direction defined by said fiber gain medium, said fiber segments disposed in said optical resonator to provide an optical conduit between said first and second reflective elements.
 18. A fiber laser as in claim 17, further comprising a fiber stretcher engaged to at least one of said fiber gain medium and said fiber segments, wherein said fiber stretcher is operable to change said pulse repetition rate by stretching said one fiber.
 19. A fiber laser as in claim 18, wherein said fiber stretcher includes a piezo element.
 20. A fiber laser as in claim 18, further comprising:a photo sensor receiving a portion of said optical pulses from said resonator; and an electronic control, communicating with said photo sensor and operating to determine said pulse repetition rate from a signal produced by said photo sensor and to produce an error signal indicative of a difference between said pulse repetition rate and a reference clock rate, wherein said electronic control is connected to control said fiber stretcher to adjust said pulse repetition rate in response to said error signal.
 21. A fiber laser as in claim 17, further comprising a positioner engaged to one of said first and second reflective elements to tune said pulse repetition rate by changing the total optical length of said resonator.
 22. A fiber laser as in claim 21, further comprising:a photo sensor receiving a portion of said optical pulses from said resonator; and an electronic control, communicating with said photo sensor and operating to determine said pulse repetition rate from a signal produced by said photo sensor and to produce an error signal indicative of a difference between said pulse repetition rate and a reference clock rate, wherein said electronic control is connected to control said positioner to adjust said pulse repetition rate in response to said error signal.
 23. A fiber laser as in claim 17, further comprising a wavelength tuning element disposed in said resonator to vary said laser wavelength within a spectral gain profile of said fiber gain medium.
 24. A fiber laser as in claim 23, wherein said tuning element includes a bandpass filter that transmits photons at said laser wavelength and absorbs photons at said pump wavelength, said bandpass filter operable to change said laser wavelength.
 25. A fiber laser as in claim 24, wherein said bandpass filter includes an interference filter or a birefringence filter.
 26. A fiber laser as in claim 17, further comprising an adjustable optical mount free of ball bearing.
 27. A fiber laser as in claim 17, further comprising a grating to select said laser wavelength within a spectral gain profile of said fiber gain medium.
 28. A fiber laser as in claim 27, wherein said grating is a fiber grating.
 29. A fiber laser as in claim 17, wherein said optical coupler is positioned in an optical path between said fiber gain medium and said semiconductor absorber to direct said pump beam towards said fiber gain medium.
 30. A fiber laser as in claim 17, further including a pump light source coupled to said optical coupler and configured to produce said pump beam.
 31. A fiber laser as in claim 30, wherein said pump light source includes a LED.
 32. A fiber laser as in claim 30, wherein said pump light source includes a laser diode.
 33. A fiber laser as in claim 17, wherein said fiber gain medium is doped with rare-earth ions.
 34. A fiber laser as in claim 33, wherein said rare-earth ions include Er ions, said laser wavelength is about 1.55 μm.
 35. A fiber laser as in claim 33, wherein said fiber gain medium is doped with a mixture of Er and Yr ions and said pump wavelength is about 1.05 μm.
 36. A fiber laser as in claim 17, wherein said optical coupler includes a dichroic beam splitter.
 37. A fiber laser as in claim 36, wherein said semiconductor absorber is configured to have a linear absorption at said laser wavelength from about 2% to abut 90%.
 38. A fiber laser as in claim 37, wherein said semiconductor absorber is configured to have a linear absorption at said laser wavelength from about 25% to abut 75%.
 39. A fiber laser as in claim 17, further comprising means for reducing light at said pump wavelength that reaches to said saturable absorber.
 40. A fiber laser as in claim 17, wherein said first reflective element is a reflector having a high reflectivity at said laser wavelength and a low reflectivity at said pump wavelength.
 41. A fiber laser as in claim 40, further comprising a lens disposed relative to said reflector to couple optical energy to and from said reflector.
 42. A fiber laser as in claim 40, wherein said first reflective element is a reflective coating directly formed on an end of said fiber gain medium, said coating having a high reflectivity at said laser wavelength and a low reflectivity at said pump wavelength.
 43. A fiber laser as in claim 17, wherein said second reflective element is a reflector having a high reflectivity at said laser wavelength and a low reflectivity at said pump wavelength.
 44. A fiber laser as in claim 43, wherein said saturable absorber is engaged to said second reflector.
 45. A fiber laser as in claim 44, further comprising a lens disposed relative to said saturable absorber to couple optical energy to and from said saturable absorber.
 46. A fiber laser as in claim 44, wherein one of said fiber segments is directly coupled to said semiconductor absorber to couple optical energy to and from said absorber.
 47. A fiber laser as in claim 17, wherein a linear absorption of said absorber and a total group velocity dispersion inside said resonator are configured so that said optical pulses are soliton pulses.
 48. A fiber laser as in claim 17, further comprising an output optical coupler disposed in said resonator to produce an output.
 49. A fiber laser with a linear optical resonator, comprising:an optical resonator having a reflective grating positioned as a first end of said resonator and configured to selectively reflect light of certain wavelengths, and an optical reflector positioned as a second end of said resonator to provide optical feedback and to have a high reflectivity at a laser wavelength; a fiber gain medium having optical transitions operable to absorb photons at a pump wavelength in a specified wavelength range and to emit photons at said laser wavelength, said fiber gain medium configured to maintain light polarization in a specified polarization direction perpendicular to an optic axis of said fiber gain medium; a semiconductor absorber disposed in said optical resonator and configured to exhibit a saturable intensity-dependent absorption and to have a bandgap equal to or less than a photon energy corresponding to said laser wavelength, said semiconductor absorber operable to lock a plurality of oscillating longitudinal modes in phase to produce optical pulses of said laser wavelength at a pulse repetition rate; an optical coupler, disposed relative to said fiber gain medium and configured to couple a pump beam at said pump wavelength to said fiber gain medium; and a plurality of polarization-maintaining fiber segments having a principle axis aligned with said specified polarization direction defined by said fiber gain medium, said fiber segments disposed in said optical resonator to provide an optical conduit between said first and second reflective elements.
 50. A fiber laser as in claim 49, wherein said reflective grating is a fiber grating.
 51. A fiber laser as in claim 49, further comprising a resonator control element which is positioned in said resonator and is configured to change the optical length between said reflective grating and said reflector, thus changing said pulse repetition rate.
 52. A fiber laser as in claim 51, wherein said resonator control element includes a fiber stretcher which is disposed to change a length of one of said fiber gain medium and said fiber segments.
 53. A fiber laser as in claim 51, wherein said resonator control element includes a positioner engaged to one of said reflective grating and said reflector.
 54. A fiber laser as in claim 51, further comprising:a photo sensor receiving a portion of said optical pulses from said resonator; and an electronic control, communicating with said photo sensor and operating to determine said pulse repetition rate from a signal produced by said photo sensor and to produce an error signal indicative of a difference between said pulse repetition rate and a reference clock rate, wherein said electronic control is connected to control said resonator control element to adjust said pulse repetition rate in response to said error signal.
 55. A fiber laser as in claim 49, further comprising a polarizer located in an optical path between said reflective grating and said reflector to define a polarization of said optical pulses, wherein said polarizer is aligned with said specified polarization direction defined by said fiber gain medium.
 56. A fiber laser as in claim 49, wherein said reflective grating is configured to tune said laser wavelength by changing an orientation of said grating with respect to an optical axis of said resonator.
 57. A fiber laser as in claim 56, further including a lens disposed relative to said reflective grating to image an optical pulse to said grating.
 58. A fiber laser as in claim 56, further including a an optical collimator disposed relative to said reflective grating to couple said optical pulses to and from said reflective grating.
 59. A fiber laser as in claim 49, wherein said fiber grating medium has an angle-polished end to reduce optical reflection thereof.
 60. A fiber laser as in claim 49, wherein said optical coupler is positioned in an optical path between said fiber gain medium and said semiconductor absorber to direct said pump beam towards said fiber gain medium.
 61. A fiber laser as in claim 49, further including a pump light source coupled to said optical coupler and configured to produce said pump beam, wherein said pump light source includes a LED or laser diode.
 62. A fiber laser as in claim 49, wherein said fiber gain medium is doped with rare-earth ions.
 63. A fiber laser as in claim 49, wherein said semiconductor absorber is configured to have a slow saturation process with a low saturation intensity and a fast saturation process with a high saturation intensity so that said optical pulses are initiated by said slow saturation process and further modified by said fast saturation process.
 64. A fiber laser as in claim 49, wherein said semiconductor absorber is engaged to said reflector.
 65. A fiber laser as in claim 64, wherein said reflector is formed of multiple semiconductor layers.
 66. A fiber laser as in claim 49, wherein one of said fiber segments is directly coupled to said semiconductor absorber to couple optical energy to and from said absorber.
 67. A fiber laser as in claim 49, further comprising a lens disposed in an optical path between said fiber gain medium and said semiconductor absorber to couple optical energy to and from said absorber.
 68. A fiber laser as in claim 49, wherein a linear absorption of said absorber and a total group velocity dispersion inside said resonator are configured so that said optical pulses are soliton pulses.
 69. A fiber laser as in claim 49, further comprising an output optical coupler disposed in said resonator to produce an output.
 70. A method of constructing and operating a fiber laser, comprising:forming a linear optical resonator by using two reflective elements; disposing a fiber gain medium in an optical path linking said two reflective elements to provide a gain equal to or greater than a total optical loss in said resonator, wherein said fiber gain medium is formed of polarization-maintaining fiber; using a saturable absorber in said resonator to mode lock multiple oscillating longitudinal modes to produce optical pulses; and configuring a linear absorption of said saturable absorber and a total group velocity dispersion inside said resonator to make said optical pulses as soliton pulses.
 71. A method as in claim 70, further comprising configuring said absorber absorber to have a slow saturation process with a low saturation intensity and a fast saturation process with a high saturation intensity; using said slow saturation process to initiate said optical pulses; and using said fast saturation process to modify said optical pulses.
 72. A method, comprising:forming a linear optical resonator by using two reflective elements; disposing a fiber gain medium in an optical path linking said two reflective elements to provide a gain equal to or greater than a total optical loss in said resonator, wherein said fiber gain medium is formed of polarization-maintaining fiber; using a saturable absorber in said resonator to mode lock multiple oscillating longitudinal modes to produce optical pulses, wherein said satuarable absorber has a slow saturation process with a low saturation intensity and a fast saturation process with a high saturation intensity; coupling a pump beam into said resonator at a location between said saturable absorber and said fiber gain medium to direct said pump beam towards said fiber gain medium and to excite said fiber gain medium; using said slow saturation process to initiate said optical pulses; and using said fast saturation process to modify a property of said optical pulses. 